Subscription-mode billing was consuming quota at TotalCost (raw) instead of
ActualCost (TotalCost * RateMultiplier), so per-group rate multipliers —
including free subscriptions (multiplier = 0) — were silently ignored.
Switch the three subscription cost writes in buildUsageBillingCommand,
finalizePostUsageBilling, and the legacy postUsageBilling fallback to
ActualCost, and add a table-driven test covering 2x / 0.5x / free multipliers
plus a balance-mode regression check.
